BUG#50780: 'show binary logs' debug assertion when binary

logging is disabled
      
The server would hit an assertion because of a DBUG violation.
There was a missing DBUG_RETURN and instead a plain return
was used.
      
This patch replaces the return with DBUG_RETURN.

mysql-test/r/no_binlog.result

+SHOW BINARY LOGS;
+ERROR HY000: You are not using binary logging

mysql-test/t/no_binlog.test

+# BUG#50780: 'show binary logs' debug assertion when binary logging is disabled
+
+-- error ER_NO_BINARY_LOGGING
+SHOW BINARY LOGS;

sql/sql_repl.cc

@@ -1606,7 +1606,7 @@ bool show_binlogs(THD* thd)
   if (!mysql_bin_log.is_open())
   {
     my_message(ER_NO_BINARY_LOGGING, ER(ER_NO_BINARY_LOGGING), MYF(0));
-    return 1;
+    DBUG_RETURN(TRUE);
   }
 
   field_list.push_back(new Item_empty_string("Log_name", 255));